Interior scene has a lot of noise, even with optimized settings
The "correct" way to reduce noise is to increase the sample size. You mentioned that this didn't completely solve the problem, but I would use the largest sample size you can reasonably afford (within your time constraints, patience, etc.). Also, I've heard that it's best to use powers of 2 for sample sizes. So maybe try 2048 instead of 2056...

Additionally, the latest versions of Blender have the denoise feature, which works better than an external noise reducer, because it has access to data not present in the final image. It will further reduce the noise in your image.



Regarding portals:


  They work best when most of the light in your scene comes indirectly from the environment through small windows. If your scene has large windows, uses many lamps or contains lots of complex materials, portals might actually slow the render down and introduce more noise.


(https://www.blenderguru.com/tutorials/using-portals-accelerate-render-times)

It seems that your windows could qualify as large (though you don't seem to have extremely complex materials), so maybe try a render without portals, and comparing the two in terms of noise and render time. I don't know which way will work best for your scene.



Finally, you could cheat and make the blinds in the windows invisible to all rays except camera rays (the opposite to this answer). This will make the result less physically accurate, but it will let in more light, and reduce the number of bounces light needs to get inside, resulting in a brighter and less noisy picture.